This invention relates to an improved motor control means and more particularly, to an improved motor control means for a two-speed single phase refrigerant compressor motor.
Conventional single phase alternating current induction motors for driving refrigerant compressors are connected to a source of power by a line contactor. In circuit with the main motor winding and the start winding are a run capacitor and a start capacitor. A start relay including a coil in parallel with the motor start winding or windings and normally closed start relay contacts in series with the start capacitor is provided. When the contactor is energized, its contacts close. The start relay coil senses the start winding voltage and is energized at a predetermined value as the start winding voltage increases during motor acceleration. Upon attainment of the predetermined value and energization of the start relay coil, the start relay contacts open. The start capacitor is operatively removed from the circuit. Assuming that the start relay is properly sized for the application, it will stay energized during all expected load and voltage conditions.
When the line contactor is deenergized and power is removed from the motor circuit, the main winding and start winding voltages drop to zero. The start relay coil is deenergized and the start relay contacts close. If the start relay coil should happen to open when the run capacitor is fully charged, there is a large amount of energy stored in the run capacitor that can be discharged into the start capacitor when the start relay contacts close. There is sufficient energy to weld the start relay contacts and thus disable the relay for the next operation.
The start relay contact is not welded closed in practice since there is an electrical path through the main winding and start winding which allows the run capacitor to discharge before the start relay contacts close. It has been found that the start relay contacts will close in about 22 milliseconds normally and a minimum measured closure time is 17 milliseconds. The time for the run capacitor to discharge through the main winding and start winding and run capacitor in a 5-ton compressor application can be computed as follows:
t = RC = (R main winding + R start winding)(C run capacitance) = (0.3 + 2.0)(55 .times. 10.sup.-6) = 2.3 .times. 55 .times. 10.sup.-6 = 0.127 milliseconds to discharge to 37 percent of the original voltage level.
From the foregoing, it is seen that the run capacitor will be discharged long before the start relay contacts close (0.127 milliseconds being much less than 17 milliseconds).
Applying the conventional single phase single speed motor control to a single phase two speed motor results in an arrangement lacking a discharge path for the run capacitor and therefore, the run capacitor can discharge through the start relay contacts to the start capacitors and thus weld the start relay contacts closed.
An object of this invention is to provide an improved motor control for single phase two speed motors which will overcome the problem of the welding of the start relay contacts in an effective and relatively inexpensive fashion.
Another object of this invention is to provide an improved motor control for a single phase two speed motor for a refrigerant compressor which incorporates a negative temperature coefficient thermistor for properly and economically controlling the discharge of the run capacitor so as to prevent welding of the start relay contacts.
Other objects and advantages of the present invention will be made more apparent hereinafter.